"He's one of my best employees. He always puts in ten-hour days, sometimes much more." Is this how your boss judges you and your colleagues? Probably yes, according to a 2010 study published in Human Relations.

In the study, a group of researchers led by business professor Kimberly Elsbach conducted extensive interviews of 39 corporate managers. They found that these managers generally considered their employees who spent more time in the office to be more dedicated, more hardworking, and more responsible.

At first glance, this seems perfectly reasonable. Hourly wages and the classic 40-hour work week have trained us to measure our labor by the number of hours we log. However, this mindset is dead wrong when applied to today's professionals. The value of lawyers, consultants, and analysts isn't the time they spend, but the value they create through their knowledge.

Even worse, when managers judge their employees' work by the time they spend at the office, they impede the development of productive habits. By focusing on hours worked instead of results produced, they let professionals avoid answering the most critical question: "Am I currently using my time in the best possible way?" As a result, professionals often use their time inefficiently.

Business meetings are a perfect illustration. Very few professionals would say that attending meetings is the best use of their time. In one survey, white-collar workers estimated that two thirds of meeting time is pure waste. I agree: all too often, information is repeated or the discussion goes off-topic.

Yet, many meetings are too long, too large, and too unfocused. Why? Consider one manager's description of an employee, as reported in Elsbach's study:

"So this one guy, he's in the room at every meeting. Lots of times he doesn't say anything, but he's there on time and people notice that. He definitely is seen as a hardworking and dependable guy."

In other words, this manager praised his or her employee not for the value that he added to the meetings that he attended, but merely for his physical presence. Given this structure of rewards, it is no surprise that we keep seeing unnecessary and unproductive meetings.

More broadly, many professionals use their time inefficiently because their firm's hour-oriented culture hasn't forced them to think rigorously about what's really important. Sometimes, this leads professionals to spend an inordinate amount of time perfecting one particular task - say, the formatting of an internal presentation - instead of spending time where it might be more useful.

Worst of all, if you measure your productivity by time spent, your only way to get ahead is to spend more hours in the office - to the detriment of the rest of your life. In research published in HBR in 2006, Sylvia Ann Hewlett and Carolyn Buck Luce reported that 62 percent of high-earning individuals in America (whom they define as the top 6% of earners) work 50 hours or more per week; 35 percent work 60 hours or more per week.

That fits my observation of New York law firms, where associates routinely bill 3,000 hours each year. That equates to 60 hours per week during a 50 week year; including non-billable hours, these 3,000-hour lawyers generally worked 12 hour days, six days a week. They barely had enough time for sleeping - let alone caring for their families, or just having fun.

What You Can Do About It

How can you remove yourself from this treadmill of long, wasted hours at work? Start by constantly evaluating your use of time - even if your organization's culture doesn't force you to.

That means knowing what's important to you, your organization, and your boss - and, vitally, what's not important. So think critically and rigorously about your priorities.

Then, be prepared to say "no" to requests that don't matter:

Decline meetings, whenever you can.To be polite, you can explain your workload and request to see the meeting's minutes instead.

Don't be afraid to use the "delete" button when reviewing your inbox.

If you can't say "no" to a certain request, recognize that it may only require a B effort. Don't spend hours bumping it up to an A unless you really need to.

While individual employees can change their own habits, organizations need strong-willed leaders to make more radical changes. These leaders must thoroughly reform their organization's implicit and explicit reward structure. Are employees praised for coming in on Saturday - even if only to finish work that could have been completed during regular hours? Are employees suspicious of others who leave early for the day in order to watch their child's Little League games?

Of course, this change won't come easily. It's easy to count hours. It's much harder to set project metrics or make subjective evaluations. But smart leaders realize that the only way they can succeed is by getting the most out of their employees. And the only way they can get the best out of their employees is to focus on results, not hours.